The present invention relates to a guided wave spatial filter, in particular but not exclusively to a guided wave spatial filter suitable for use in conjunction with rib waveguides in optical integrated circuits.
For the purposes of describing the present invention, optical radiation is defined as radiation having a free-space wavelength in a range of 100 nm to 15 μm.
Monolithic optical integrated circuits are well known. They are described in a publication “Introduction to Semiconductor Integrated Optics” by H P Zappe (ISBN 0-89006-789-9, Artech House Publishers 1995). A conventional optical integrated circuit includes a substrate onto which one or more optical waveguides are fabricated, the waveguides operable to guide optical radiation around the circuit. It is found especially convenient in such a circuit to employ surface rib waveguides, the rib waveguides being susceptible to fabrication using planar fabrication processes such as lithography and chemical etching.
A rib waveguide assumes the form of a ridge projecting from a major surface of the substrate, the ridge having associated therewith one or more refractive index interface regions to assist optical radiation confinement in the ridge. The optical radiation is guided along the ridge and is confined primarily to a region lying between the one or more interface regions and an upper surface of the ridge remotest from the substrate. In such a rib waveguide, a plurality of interface regions gives more degrees of freedom when optimising optical characteristics of the waveguide.
The ridge is designed to be of a width and height comparable to within an order of magnitude with the wavelength of optical radiation to be guided along the rib waveguide formed by the ridge. For example, a rib waveguide designed to guide optical radiation having a free-space wavelength of substantially 1.5 μm will have a lateral width in the order of 3 μm and a projection height of its ridge above its substrate in the order of 1 μm. The rib waveguide can be fabricated from silicon or alternatively from III–V compounds, for example aluminium gallium arsenide (AlGaAs) and gallium arsenide (GaAs). III–V compounds such as AlGaAs possess a crystal structure lacking a centre of symmetry and therefore exhibit both Pockets and Kerr effects. As a consequence of such asymmetry, rib waveguides based on III–V compounds are especially suitable for use in fabricating active optical integrated circuits, for example optical modulators and switches.